I know it's crazy to build one, but I have found a great new project to
start. A spectrum analyzer. The first one I build will probably work from
0~200 MHz. I have designed an excellent PLL that delivers quite acceptable
performance.

The part I am having to think about is how to accurately measure the final
dB signal level. Here is the situation:

For those of you into RF: The analyzer is a built as a AM receiver with a
455kHz final IF.

Should I run the IF into a Full wave rectifier, filter it, then feed the
voltage to the PIC via a high-res D/A?

-or-

Should I run the IF into a half wave rectifier, then to the D/A, and let the
PIC analyze the "Pulse" to determine it's maximum voltage?

Or am I going about this wrong? I just want to know the amplitide of a
455kHz signal, with precision.

On Thu, Feb 03, 2000 at 07:44:05PM -0600, Anthony Clay wrote:
> The part I am having to think about is how to accurately measure the final
> dB signal level. Here is the situation:
>
Hmmm.... I remember reading somewhere an article in a trade rag that showed
using some derivative of an AGC circuit to measure signal output. If I can
find the article in my archives (fat chance) I'll re-read it and post what
I find.

I would be tempted to use the full wave rectifier approach, using a filter
cap to smooth it out, and sampling. The trick here as I recall is to keep
the value of the cap sufficiently large enough to do the filtering but not
so large as to cause a "rubber screwdriver" effect.

On the other end of the scale is a problem of aliasing. If your sample is
taken at the wrong time you could be reading "off peak" values for amplitude
and therefore not the real amplitude. Of course this is where your filter
cap comes in.

It's been a while since I have thought about these subjects so I may well be
off here....

>On Thu, Feb 03, 2000 at 07:44:05PM -0600, Anthony Clay wrote:
> The part I am having to think about is how to accurately measure the final
> dB signal level.

Do a search on 'RSSI'. Both AD and Phillips make chips that give a
reasonably good log response to input level.

You might also want to search on 'poor mans spectrum analyser' - the author
shows how to use TV tuners as the front end of a low cost unit. His log
detector is based upon a chain of cmos invertors operating in the linear
region with a resistive sum of the individual outputs. As each stage
saturates, it quits adding to the output. It seems reasonably accurate, is
incredibly cheap, and uses parts made by many suppliers.